Semiconductor chips typically have gate oxide layers of a single thickness. However, it is sometimes preferable to have gate oxide layers of varying thicknesses on a single chip. For example, input-output drivers normally are subjected to a higher supply of voltage entering externally to the chip. The use of thicker gate oxides would prevent these devices from having unacceptably short time dependent dielectric breakdown (TDDB) characteristics, and also reduce the possibility of needing to add circuitry to the chip to avoid early TDDB failure.
Current methods require a complicated procedure in order to produce a semiconductor chip having varying gate thicknesses. First, a gate oxide layer is grown on the semiconductor chip surface. A portion of this gate oxide layer must then be masked. The mask can be used either to slow the growth of the gate oxide layer on the masked portion of the chip surface or to isolate the masked area while the unmasked portion of the gate oxide layer is further reduced.
Two major problems are associated with the use of this masking technique to produce different gate oxide thicknesses on a single chip. First, the use of the mask can cause contamination problems in the gate oxide layer. Any impurities in the gate oxide layer may affect the performance of the gate oxide layer and consequently the performance of the entire chip. Second, the use of a masking technique requires additional process steps which are costly and time-consuming. Accordingly, there is a need for a process of controlling gate oxide thickness in the fabrication of semiconductor devices which overcomes the above-mentioned deficiencies.